Neurosurgery notes/Paediatrics/Developmental abnormalities/Intracranial Anomalies/Posterior Fossa Cystic Malformations/Persistent Blake's pouch cyst

Persistent Blake's pouch cyst

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4th ventricle size
enlarged
Status
Done
Communication between 4th and cisterna magna
No
Files & media
Untitled picture.pngUntitled picture.png
Hydrocephalus
Yes
Image explain
Up: • CSF collection in the fourth ventricle that is contiguous (arrow) • A collection inferior to the cerebellum (asterisk). • Small arrowheads show upward mass effect from fluid.   Down: • Enlargement of 4th ventricle, which communicates with an infravermian cystic compartment (arrow) corresponding to enlargement of the Blake pouch; • a normal vermis; • and supratentorial hydrocephalus (arrowhead).
Intrathecal contrast CT
Communicates with ventricles
Inferior Vermis
Normal
Mass effect on the cerebellar hemisphere
No
Occipital bone scalloping
No
Position of choroid plexus
displaced into the superior cyst wall
Vermis remnant
Normal
cerebellar hemispheres
Normal
post fossa size
Elevation of tentorium but with a normally positioned torcula

Normal 4th ventricular formation

  • Blake’s pouch
    • aka rudimental fourth ventricular tela choroidea
    • is a normal transient structure during embryological development
    • Regresses at 12 weeks of gestation
    • Though a fenestration to form the foramen of Magendie (which forms in up to the fourth month of gestation).
      • The foramina of Luschka open later than the foramen of Magendie during the embryologic development
  • Embryonic sequence of events in the development of the roof of the fourth ventricle.
  • The plica choroidea (choroid plexus) divides the roof of the fourth ventricle into an anterior membranous area and a posterior membranous area (a).
  • The cerebellar vermis originates from the anterior membranous area (b), which eventually disappears.
  • Blake's pouch appears as a protrusion of the posterior membranous area of the fourth ventricular roof (c), which later communicates with the subarachnoid space forming the foramen of Magendie (d).
AMA: Anterior membranous area, C: Cerebellum, CP: Choroid plexus, IV: Fourth ventricle, PMA: Posterior membranous area.
AMA: Anterior membranous area, C: Cerebellum, CP: Choroid plexus, IV: Fourth ventricle, PMA: Posterior membranous area.
 

Definition

  • Ballooning of the inferior medullary velum due to an imperforated foramen of Lushka and magendie.

Mechanism

  • Absence in fenestration of the Blake's pouch

Numbers

  • Rare

Clinical presentation

  • Asymptomatic
    • If there is adequate flow through foramen of Lushka
  • hydrocephalus (usually headache, vomiting, blurred or double vision)
    • More likely to be present in blake pouch cyst rather than mega cisterna magna.
    • MRI cannot differentiate Mega cisterna magna from persistent blake pouch cyst
  • Macrocephaly

Imaging

  • MRI
      • Infravermian cyst that communicates with fourth ventricle (or a 4th ventricular diverticulum)
        • choroid plexus can extend from fourth ventricle → superior portion of cyst,
        • cyst is smooth with thin walls that can be visualised on thin sagittal T2 images
        • it can impress on medial side of cerebellar tonsils due to size
        • cyst does not communicate with the cisterna magna posteriorly
        • upward displacement of the vermis
      • Elevation of tentorium but with a normally positioned torcula
        • Torcula is elevated in dandy walker
      • CISS:
        • can see a membrane at the craniocervical junction
      notion image
       
    • no vermian hypoplasia or rotation
    • presence of a cyst in a retrocerebellar or infraretrocerebellar location, which is essentially a diverticulum of the consequently enlarged fourth ventricle
      • choroid plexus in a Blake pouch cyst at times may be identified as being displaced into the cyst along its superior wall (under and posterior to the vermis),
      • The displacement of the choroid plexus is best visualized as an enhancing structure on sagittal contrast material–enhanced T1-weighted images
      • The consistent presence of hydrocephalus allows the differentiation of Blake pouch cyst from mega cisterna magna.
    • Mild mass effect may result in indentation of the inferior vermis or of the caudal and medial aspects of the cerebellar hemispheres.
    • The posterior fossa is typically normal in size.
    • Supratentorial morphologic abnormalities other than hydrocephalus are usually absent.
      • Hydrocephalus of all 4 ventricles

Treatment

  • ETV